In the past, chlorinated biphenyls have seen wide use as dielectric liquids in electrical equipment. The polychlorinated biphenyls are particularly well suited in applications where fire hazards are a problem because of their relatively high fire point. While the polychlorinated biphenyls produce an effective dielectric system for electrical equipment, their usage has provided certain ecological problems, in that the polychlorinated biphenyls are virtually non-biodegradable, with the result, that if leakage or rupture occurs in the electrical equipment, or if the equipment is discarded as obsolete, the polychlorinated biphenyl will remain as a pollutant in the environment and will not degrade to any appreciable extent over extended periods of years.
More recently, a mixture of a mono-halogenated diphenyl oxide and a mono-halogenated alkyl diphenyl oxide, as disclosed in U.S. Pat. No. 4,097,912 has been used as a dielectric liquid for a capacitor. The dielectric system, as disclosed in the aforementioned patent has improved corona characteristics and low dielectric losses and is substantially biodegradable.
U.S. Pat. Nos. 4,054,937 and 3,796,934 disclose liquid dielectrics for capacitors utilizing an alkyl diphenyl. In U.S. Pat. No. 3,796,934, the alkyl diphenyl is combined with a diaryl sulfone, while U.S. Pat. No. 4,054,937 discloses a dielectric liquid composition composed of a mixture of mono and di alkyl derivatives of diphenyl, diphenyl oxide or diphenyl methane.